There are various types of angular velocity sensors (gyro sensors) depending on the method of operation. Known types of angular velocity sensors include: mechanical type that utilizes the precession of a body of rotation; optical type that utilizes change in the timing of light reception due to the rotation of laser light that is rotated in an enclosure; and fluid type wherein a jet of gas for sensing is directed at a heat ray, and change in jet quantity due to the rotation of an enclosure is detected through heat ray temperature. Recently, demand for angular velocity sensors for vehicle direction detection in car navigation systems or the like has been rapidly increased. As a result, vibration type angular velocity sensors that are inexpensive and lightweight as compared with the above-mentioned types are going mainstream. The vibration type angular velocity sensor is so constructed that the following takes place: when angular velocity is exerted on vibrators that vibrate in a predetermined reference direction, a new vibration component is detected. This new vibration component (hereafter, referred to as “angular velocity vibration component”) is based on Coriolis force in the direction of detection orthogonal to the reference direction. Based on the vibration component, angular velocity information is outputted.
Conventionally, the following are publicly known as systems wherein vehicle control is carried out using an angular velocity sensor: vehicle stability control system wherein skid of a vehicle is detected, and the brake and torque on each wheel are optimally controlled to keep the vehicle in normal conditions; four-wheel steering angle control system wherein the steering angle of the front wheels or rear wheels of a vehicle is controlled; and the like. These types of systems use an angular velocity sensor to detect abnormal conditions of vehicles such as slide. Thus, it is demanded to enhance the reliability of the angular velocity signal.
Vibration type angular velocity sensors are characterized by the following: when translational acceleration is applied to a vehicle during angular velocity detection, it is superposed as noise on angular velocity detection wave form based on Coriolis force. The technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-21517 adopts the following method: two sets of vibration type sensor units that are driven in opposite phases are combined together, and the outputs of the sensor units produced in opposite phases are differentially amplified; the acceleration component is thereby canceled. Japanese Unexamined Patent Application Publication No. 2003-21517 discloses the following as a common means for use in vehicle control: the output system of an angular velocity sensor is divided into main and sub; the sub output is used as backup for the main output. When any trouble occurs in either output system, it can be detected by comparing the output of the sub output system and that of the main output system.
The technology disclosed in Japanese Unexamined Patent Application Publication No. 2003-21517 adopts the following method for canceling acceleration: the outputs of two sensor units are synthesized beforehand, and the synthesized output is divided into sub output and main output. However, this method involves a problem. The first and second sensor units form the basis of sensor output. When any trouble occurs in a component (e.g. a vibrator or an electrode for vibration detection) internal to the sensor units, it cannot be detected. This is because a particular difference is not produced between sub output and main output unless the output system is faulty. Thus, it is required for the vibration type angular velocity sensor to detect anomalies with ease and reliability even when any trouble occurs in individual sensor units.
Further, if the driving amplitude of a vibrator gets out of a predetermined range in a vibration type angular velocity sensor, some anomaly can occur in the zero point or the sensitivity of angular velocity sensor output. Therefore, it is required to detect the driving amplitude and determine whether it is within the predetermined range or not. Specifically, the following operation is performed as disclosed in Japanese Unexamined Patent Application Publication No. 2000-88578: the driving amplitude of vibrators is detected with piezoelectric elements, and is subjected to charge-voltage conversion. Further, rectification is carried out, and the rectified signal is used as an amplitude monitoring signal. Anomaly detection is carried out based on whether the level of this amplitude monitoring signal is within a predetermined range or not.
If translational acceleration is applied to a vehicle during angular velocity detection, the following occurs in a vibration type angular velocity sensor: the translational acceleration is superposed as noise on angular velocity detection waveform based on Coriolis force. The technology disclosed in Japanese Unexamined Patent Application Publication No. 2000-88578 adopts the following method: two sets of vibration type sensor units that are driven in opposite phases are combined, and the outputs of the sensor units produced in opposite phases are differentially amplified. Thus, the acceleration components are canceled out. In this case, the vibrators of the two units must be synchronously driven. In amplitude control at a vibration driving unit, amplitude monitoring signals individually taken out of the individual units are added in phase, and the sensitivity for amplitude monitoring signal is thereby enhanced. The amplitude monitoring signals obtained as the result of addition are also used in anomaly detection.
If any anomaly occurs in a sensor unit, the following takes place in the amplitude monitoring signal obtained as the result of addition: anomalous amplitude components are superposed on fiducial amplitude expected in normal operation. When two sensor units are combined and used, as mentioned above, a problem arises. The following procedure must be taken if the amplitude monitoring signal obtained as the result of addition is used for anomaly detection: as illustrated in the left part of FIGS. 12A to 12H, an anomalous vibration component α must be detected, together with a fiducial amplitude component A twofold amplified, with the same signal amplification factor G. However, there is a limitation on the signal amplification factor G for the reason of the operating voltage of the circuit. A restriction is imposed on the allowable margin for superposition of the anomalous amplitude component α to the fiducial amplitude A. Thus, anomalies cannot be detected with accuracy.